ECG Variations in Hypothyroidism
Hypothyroidism classically displays bradycardia, low-voltage QRS complexes, T wave inversion or flattening, and prolonged QT interval on electrocardiogram.
Primary ECG Findings
Based on authoritative guideline evidence, the most common electrocardiographic manifestations of hypothyroidism include 1:
- Bradycardia - The most frequently observed finding, reflecting decreased metabolic demands
- Low-voltage QRS complexes - Present in approximately 25% of cases 2
- T wave abnormalities - Including inversion or flattening, with the characteristic "mosque sign" (dome-shaped symmetric T wave in the absence of ST segment) 3
- Prolonged QTc interval - Correlates with TSH severity 4, 5, 6
Additional ECG Abnormalities
Beyond the classic findings, hypothyroidism may present with:
- Prolonged PR interval - First-degree AV block can occur 4, 2
- Increased QRS duration 4
- Narrowed pulse pressure (clinical finding that accompanies ECG changes) 7
Correlation with Disease Severity
The severity of ECG changes correlates directly with serum TSH levels 6. Patients with TSH >10 µIU/mL demonstrate significantly more pronounced ECG abnormalities compared to those with milder elevations 6. The QTc interval shows positive correlation with rising TSH (r = 0.41, p = 0.002), while heart rate demonstrates negative correlation (r = -0.53, p < 0.001) 6.
Age and Sex Considerations
Recent research reveals important demographic variations 4:
- Women show more pronounced bradycardia and shorter QTc intervals with hypothyroidism
- Older patients demonstrate less pronounced low-voltage findings
- Age significantly modifies the association between thyroid dysfunction and heart rate/QTc changes
Clinical Pitfalls to Avoid
Do not attribute low-voltage complexes solely to pericardial effusion. While pericardial effusion occurs in 5-30% of hypothyroid patients 1, low voltage correlates more strongly with decreased free T4 and lower serum albumin (reduced colloid osmotic pressure) rather than the presence of effusion itself 8. The two findings can coexist but have independent mechanisms.
Recognize that ECG changes are reversible with treatment. L-thyroxine replacement significantly reduces QT dispersion (45±23 vs. 30±13 msec, p=0.008) and QTc dispersion (49±23 vs. 32±14 msec, p=0.005), potentially reducing arrhythmia risk 5.
Rare but Important Variant
In rare cases, hypothyroidism can present with Brugada-type ECG pattern (coved-type ST elevation ≥2mm in V1-V2), particularly in patients with SCN5A gene variants 9. This pattern resolves with thyroid hormone normalization.
Clinical Implications
Thyroid function testing is recommended in all patients presenting with unexplained bradycardia, low-voltage QRS, or heart failure 7. The ease of diagnosis through thyroid function tests and availability of definitive treatment make this evaluation essential in the workup of cardiac symptoms with compatible ECG findings.